Electrical conductors formed from mixtures of metal powders and metallo-organic decompositions compounds

ABSTRACT

The present invention relates to a thick film formed of a mixture of metal powders and metallo-organic decomposition (MOD) compounds in an organic liquid vehicle and a process for advantageously applying them to a substrate by silk screening or other printing technology. The mixtures preferably contain metal flake with a ratio of the maximum dimension to the minimum dimension of between 5 and 50. The vehicle may include a colloidal metal powder with a diameter of about 10 to about 40 nanometers. The concentration of the colloidal metal in the suspension can range from about 10 to about 50% by weight. The MOD compound begins to decompose at a temperature of approximately about 200° C. to promote consolidation of the metal constituents and bonding to the substrate which is complete at temperatures less than 450° C. in a time less than six minutes. The mixtures can be applied by silk screening, stencilling, gravure or lithography to a polymer-based circuit board substrate for producing rigid and flexible printed wiring boards in a single operation with negligible generation of hazardous wastes. The same mixtures can be used in place of solder to assemble circuits by bonding electrical components to conductors as well as to make the conductors themselves.

BACKGROUND OF THE INVENTION

The United States Government has a royalty-free license in thisinvention for its own use and the right in limited circumstances torequire the patent owner to license others as provided for by the termsof the Small Business Innovation and Research Contract N00014-94-C-0236.

1. Field of the Invention

This invention relates to electronic circuits and particularly toprinted circuits on conventional polymer-based substrates.

2. Description of the Related Art

Printed circuits are universally used to interconnect and "package"active and passive electronic components to perform a myriad of usefulfunctions. These circuits can be of the rigid type which consist oflaminated "boards" usually of glass-reinforced epoxy, but also ofpaper-phenolic and other constructions. There are also flexible circuitsbased on unreinforced polymer films such as Kapton polyamidemanufactured by DuPont. In both cases the circuits are produced by acombination of etching and deposition technologies based on the use ofphotoresists which are exposed to light and developed to permit apattern of conductors to be etched and/or plated on the substrate.

These conventional processes are described, for example, in "PrintedCircuit Board Basics", M. Flatt, Miller Freeman, San Francisco, 1992.The conventional technology requires many steps of plating, bothelectroless and electrolytic, and etching to produce a finished board.At each of these stages there is opportunity for error, and hazardouswastes are generated in many of the processes. The principal wastes canbe the result of etching away unwanted conductor material to leavebehind the desired conductor pattern, and from spent solutions fromelectrolytic plating of copper, tin and lead, as well as spentelectroless plating solutions. The latter are the most difficult tohandle because of the presence of formaldehyde reducing agent, a knowncarcinogen, as well as the toxic heavy metals.

There is a major emphasis on speed of response in the circuit boardindustry, and technology which can reduce the time to produce prototypeboards and facilitate modifications with minimum tooling change, as wellas reduce in-process inventory and manufacturing time would beadvantageous. In particular, a low temperature analog of "thick film"technology which is used to print circuit conductors onto ceramicsubstrates which are fired at high temperatures is desirable.

The ideal solution to this problem would be an ink which could beapplied only where a conductor is needed by a simple printing technologyto any type of printed circuit substrate material, and which could becured at a temperature which would not damage the substrate. Theconductor should have an electrical conductivity greater than half thatof the bulk metal and should be strongly adherent to the polymersubstrate after curing. This ink would allow the production of circuitboards at high production rates with the generation of no hazardouswaste by a simple two-step print and heat process.

The primary problem in the industry is the requirement for highelectrical conductivity with a low enough curing temperature to becompatible with the polymer-based circuit boards. Conventional solutionsto the problem provide low temperature conductive epoxies with poorelectrical conductivity and high temperature thick film inks with goodelectrical conductivity which can only be used on ceramic substrates.These small, expensive and specialized substrates can withstand therequired thick film ink firing temperatures of more than 650° C. andusually above 850° C. An ink which could duplicate this performance onpolymer-based substrates at approximately 250° to 350° C. would permitapplication of this technology broadly in the 20 billion dollarworldwide rigid circuit board industry and the one billion dollarworldwide flexible circuit industry.

"Thick film" technology, as described by R. W. Vest in "ElectronicCeramics", R. Breckenridge, ed., 1991, is routinely practiced to producehybrid circuits on ceramic substrates. The conductor patterns arecreated by silk screening or stencil printing thick film pastes or inksonto ceramic substrates and firing them at temperatures of 850° to 1100°C. to reduce the metal-containing inks to metal. An example of such inksare silver-palladium compositions described by Wang, Dougherty, Huebnerand Pepin, J. Am. Ceram. Soc. 77(12), 3051-72 (1994). Typically thickfilm inks contain metal powders, an inorganic glass binder and a vehicleconsisting of a polymer binder and a solvent. The vehicle provides thecorrect consistency for screen printing and consists typically of apolymer such as ethyl cellulose, hydrogenated rosin or polyacrylicsdissolved in a low volatility solvent. Common solvents are terpineol,dibutyl carbitol and various glycol ethers and esters. The inks areapplied to ceramic substrates by screen printing, dried to drive off thesolvent and heat treated, usually in a belt furnace, to decompose thepolymer binder and fuse the metal and the inorganic glass binder. Theglass phase provides the bond to the substrate which is usually alumina,and the metal provides the electrical conductivity. Typically theconductors have a striated cross section with layers of glassalternating with layers of metal. The glass tends to concentrate at theceramic interface and the metal at the air interface. The conductivityis typically one half to one quarter that of the bulk metal.

A number of thick film compositions contain surfactants to improvescreenability and stability of the metal powder dispersions. Often thesesurfactants are metallo-organic compounds such as soaps of carboxylicacids. These are convenient in that they decompose at relatively lowtemperature to deposit the metal or its oxide which can perform a usefulfunction in the fired conductor.

For example, U.S. Pat. Nos. 5,071,826 ('826 patent) issued on Dec. 10,1991 and 5,338,507 ('507 patent) issued on Aug. 16, 1994 to J. T.Anderson, V. K. Nagesdh and R. C. Ruby and assigned to Hewlett PackardCo. of Palo Alto, Calif. describe the addition of silver neodecanoate tosuperconducting oxide mixtures to increase the critical current formulticrystalline ceramic superconductor materials. The neodecanoate isdecomposed to the metal at 300° C. to coat the superconducting grainswith silver. The coated grains are then sintered and oxidized at600°-800° C. to produce an oxide superconductor of enhanced strength andcritical current.

The addition of titanate to thick film conductors by decomposition of anorgano- metallic titanate is described by K. M. Nair in U.S. Pat. No.4,381,945, assigned to E. I. DuPont de Nemours and Co. Wilmington, Del.

U.S. Pat. No. 4,599,277 relates to a process for adding organometalliccompounds to thick film inks to increase the densification temperatureof the metal to match that of the ceramic substrate at 850°-950° C.

Other conventional thick film paste compositions containing silverflake, glass frit and silver resinates, which are carboxylic acid soaps,as well as surfactants such as Triton X 100, are described in U.S. Pat.Nos. 5,075,262, and 5,183,784. The metal resinate was found to promoteadhesion and minimize cracks and voids in bonding semiconductor dies toa ceramic substrate at 350°-450° C.

U.S. Pat. Nos. 4,130,671, assigned to the U.S. Department of Energy,discloses a similar composition of glass frit and silver resinate whichwas decomposed at low temperature to provide silver-coated glassparticles similar to the superconductor described in the '826 and '507patents. The particles are applied to a substrate either before or afterdecomposition of the resinate and fired in an oxidizing atmosphere at500° to 700° C. to provide a conductor of metal-coated glass particles.

Still other conventional thick film compositions of glass and metalpowders in an organic vehicle but without the resinate are described inU.S. Pat. Nos. 5,250,229 and 5,378,408 assigned to DuPont, de Nemoursand Co. The above described conventional thick film compositions havethe shortcoming of requiring high temperatures, i.e., greater than about450° C. to bind the composite to the substrate.

To create a low temperature analog of the thick film process, it will benecessary to find a new mechanism to obtain adhesion and cohesion of thedeposited metal which can operate at temperatures below 450° C., whichis the extreme upper temperature limit that polymers can tolerate. Theuse of inorganic glass powder binders which are universally used inconventional thick film inks is not possible in this application becausenone of them melt at a low enough temperature, and the glass will notbond to the metal or to the polymer substrates.

Some approaches to creating electrically conductive inks for applicationto polymer substrates have been described. The most common one is thecreation of conductive epoxies or conductive inks by incorporating metalpowder, usually silver powder, in an organic matrix. This is a majorindustry with products available from Ablestik, AIT, Hokurika, M-Tech,Thermoset, Epoxy Technology and Ferro, among others. These materials canbe printed on circuit boards, and they have good adhesion. An example ofthe application of this technology was described in an article by K.Dreyfack in Electronics 52(17), 2E-4E, 1979, on Societie des ProduitsIndustrielles ITT who silk screened silver and graphite-based conductorsof this type onto rigid and flexible circuits. The problem with thisapproach is that the inks conduct by random contacts between powdergrains in the organic matrix, and the conductivity is poor. Typicalvalues of the resistivity, which is the reciprocal of conductivity, are40 to 60 microhm cm, compared to bulk silver at 1.59 microhm cm and hightemperature thick film conductors at 3-6 microhm cm. A typicalresin-bonded copper powder conductor is described in Japanese PatentApplication 52-68507, June, 1977.

U.S. Pat. No. 4,775,439, assigned to Amoco Corp., Chicago, Ill.,describes a more elaborate approach with the same results. A metalpowder and binder are applied to a substrate and dried. The trace isthen covered by a polymer film which is adhesively laminated to thesubstrate to hold the conductor in place. However, this patent does notaddress the problem of obtaining electrical conductivity comparable tobulk metal.

Near bulk conductivity has been achieved at low temperature bydecomposing metallo-organic compounds on various substrates. They can beapplied by ink jet printing as described by R. W. Vest, E. P. Tweedelland R. C. Buchanan, Int. J. (Vest et al.) of Hybrid Microelectronics 6,261-267, 1983. Vest et al have investigated so-called MOD(Metallo-Organic Decomposition) technology over many years. The mostrelevant aspect of this research was reviewed in "Liquid Ink JetPrinting with MOD Inks for Hybrid Microcircuits" Teng, K. F., and Vest,R. W., IEEE Transactions on Components, Hybrids and ManufacturingTechnology, 12(4), 545-549, 1987. The authors described their work onprinting silver and gold conductors as well as dielectrics andresistors. MOD compounds are pure synthetic metallo-organic compoundswhich decompose cleanly at low temperature to precipitate the metal asthe metallic element or the oxide, depending on the metal and theatmosphere. The noble metals, silver, gold and the platinum groupdecompose to metal films in air. The organic moiety is bonded to themetal through a heteroatom providing a weak link that provides for easydecomposition at low temperature. An oxygen bond, as in carboxylicacid-metal soaps, has been found to be satisfactory, as have amine bondsfor gold and platinum.

Vest et al. investigated metallization of ceramic substrates and siliconby ink jet printing of xylene solutions of soaps such as silverneodecanoate and gold amine 2-ethylhexanoate. Images of satisfactoryresolution (0.003 inches or 75 microns) were obtained, but theconductivity was low because of the extremely small thickness of thelayers after decomposition which was less than a micron.

Preliminary experiments by the inventor of the present application onepoxy-glass circuit boards with silver neodecanoate solutionsdemonstrated that well-bonded conductors could be produced on polymersubstrates. Again, the difficulty was that they were very thin and hadinadequate conductivity. It was found that the addition of more MODcompound resulted in wider traces but not thicker ones. The MOD compoundmelts before decomposing and spreads over the surface uncontrollably.Since melting provides for a well-consolidated metal deposit afterdecomposition, which is desirable, and since some MOD compounds areactually liquids at room temperature, this is an unavoidable problem. Apossible solution to this problem is to build up the thickness byprinting many layers, which Vest et al found suitable for metallizingsilicon solar cells, but this detracts from production of circuits in asingle pass which is the desired result.

Similar materials and techniques have been used to apply thin filmmetallization and seed coatings which are then built up with solder orelectroplating. U.S. Pat. No. 4,650,108, issued on Mar. 17, 1987, to B.D. Gallegher and assigned to the National Aeronautics and SpaceAdministration, Washington, DC; U.S. Pat. No. 4,808,274 issued on Feb.28, 1989, to P. H. Nguyen and assigned to Engelhard Corp. Menlo Park,N.J.; U.S. Pat. No. 5,059,242 issued on Oct. 22, 1991 to M. G. Firmstoneand A. Lindley and U.S. Pat. No. 5,173,330 issued on Dec. 22, 1992, toT. Asano, S. Mizuguchi and T. Isikawa and assigned to MatsushitaElectric Co, Ltd. Kadoma, Japan, are examples. However, the abovedescribed thin films alone cannot provide adequate conductivity.

A creative attempt to circumvent the resistivity problem was describedin U.S. Pat. No. 4,487,811 issued on Dec. 11, 1984, to C. W.Eichelberger and assigned to General Electric Co. Schenectady, N.Y. Thispatent describes augmenting the conductivity by a replacement reactionof metal in the deposit by a more noble metal in solution, for examplethe replacement of iron by copper. In the process of doing this, thecontact between particles is improved by the greater volume of thereplacement metal and its greater intrinsic conductivity. A resistivityof 7.5 microhm cm was achieved, substantially better than silver-loadedepoxies, but short of the performance of thick film inks. Thereplacement reaction solved yet another problem of polymer inks in thatthe material was solderable, which conductive epoxy formulations ingeneral are not.

Another approach to solderability was described in U.S. Pat. No.4,548,879 issued on Oct. 22, 1985 to F. St. John and W. Martin andassigned to Rohm and Haas Co. Philadelphia, Pa. Nickel powder was coatedwith saturated monocarboxylic acid with ten or more carbon atoms. Thecoated powder was mixed with novolac epoxy resins in a butyl carbitolacetate vehicle and silk screened onto an epoxy-glass board. Aftercuring at 165° C., the conductive trace could be solder-coated byfluxing and dipping into molten solder, while a trace made with uncoatednickel powder could not be soldered. No improvement in electricalconductivity was described with this process.

U.S. Pat. No. 4,186,244 ('244 patent) issued Jan. 29, 1980, and U.S.Pat. No. 4,463,030 ('030 patent) issued Jul. 31, 1984 to R. J. Deffeyes,and H. W. Armstrong and assigned to Graham Magnetics, Inc. NorthRichland Hills, Tex. relates to silver powder compositions having a lowfilm-forming temperature. The silver powder was formed by decomposingdry silver oxalate in the presence of a long chain carboxylic acid,either saturated (stearic acid, palmitic acid) or unsaturated (oleicacid, linoleic acid). The acid reacted with the metal powder as it wasformed to provide a protective coating on the surface and to limit theparticles to sub-micron size. The particles were washed to remove excessacid and blended with an equal weight of a conventional thick filmvehicle consisting of ethyl cellulose polymer binder and pine oilsolvent.

The resulting ink was coated on a ceramic or polyamide substrate andheated to 250° C. in air for 30-90 seconds to convert the coated powderto a silver conductor with a conductivity of one ohm per square. Thecoating is said to be solderable without flux, which is believable ifresidual acid is acting as a flux. It is stated to be resistant toleaching in a bath of molten solder, which is unexpected, based on thewell known solubility of silver in solder. The explanation may lie inthe quoted conductivity, which is a thousand-fold less than thatrequired for practical circuits.

Example I of the '244 patent indicates that the resistance of theresulting material drops markedly during the "visible fusing" period,from about 6 ohms per square to one ohm per square. The "visible fusing"period is stated to occur when the substrate is heated to 250° C. in airfor 90 seconds during which the conductor pattern changes to asilvery-white color. No thickness for the deposit is stated, which wouldallow a calculation of the resistivity of the silver. Most thick filmsare of the order of 25 microns (0.001 inch) thick, and this is generallyused as the standard for comparison. At 25 microns thickness one ohm persquare corresponds to a resistivity of 2500 microhm cm compared to theresistivity of bulk silver which is 1.59 microhm cm. This suggests avery poorly consolidated deposit with entrained nonconducting material.Even a one micron thick film corresponds to a resistivity of 100 microhmcm. A resistance of one ohm per square is far too high for practicalcircuitry which typically has traces with lengths of many hundreds ofsquares.

The unconsolidated nature of the deposit may be due in turn to the verytenacious stearate coating on the silver particles, which is an objectof the '244 and '030 patents. It is well known that stearates andsimilar materials, which are commonly added to silver and other powdersto prevent agglomeration, are very difficult to remove, even attemperatures of 625° C. and above. If not removed, they will inhibitsintering and increase resistivity. This subject was discussed in"Effect of Particle Size Distribution in Thick Film Conductors", R. W.Vest, Proceedings of the Flat Plate Solar Array Project Research Forumon Photovoltaic Metallization Systems Nov. 15, 1983 DOE/JPL-1012-92.

A somewhat similar silver flake material was described in U.S. Pat. No.4,859,241. The flake was prepared by milling silver powder with silverstearate surfactant in an organic solvent to produce silver stearate-coated silver flakes providing a glass-filled ink composition ofsuperior stability. This is a common method of preparing stable powdersand flakes of silver.

None of the materials or mixtures described above accomplish the goal ofproviding an ink which can be cured to a well-bonded, well-consolidatedmetallic conductor with an electrical conductivity comparable toconventional thick film inks but with a curing temperature belowapproximately 350° C., which is required for compatibility withconventional polymer-based circuit board substrates. None of thesematerials has made it possible to impact the circuit board industry withnew technology for rapid production by a simple process with nohazardous waste production. A new approach to provide this lowtemperature capability is needed.

SUMMARY OF THE INVENTION

Briefly described, the invention comprises a mixture of aMetallo-Organic Decomposition (MOD) compound and one or more metalpowders. The novel composition has unexpectedly been found toconsolidate to solid metal and bond firmly to conventional polymer-basedcircuit board substrates at temperatures below 350° C., at whichtemperature the substrates can survive.

Preferably, a silver flake is added to a silver neodecanoate MODcompound to immobilize it during melting and decomposition. Thismaterial restricts the spread of the liquid MOD compound so that it ispossible to screen print patterns with lines and spaces of about 0.4 mm.The resolution of the resulting conductors is such that 0.2 mm lines andspaces can be provided.

The MOD compound also improves the definition of screened circuit tracesof silver flake. It has been found that silver flake mixed withα-terpineol will sinter to reasonably well-consolidated, well-bondedtrace on polymer substrates, but that the mixture does not produce goodscreen images. The addition of the MOD compound substantially improvesthe quality of the screen image before heat treatment in addition toimproving the bonding of the silver flake during heat treatment and itselectrical conductivity afterwards.

The addition of silver flake also increased the thickness of thedeposits. It has been found that a trace formed of a pure MOD compoundwas limited to a few microns in thickness. The addition of silver flaketo the MOD compound makes it possible to produce deposits up to 50microns (0.002") or more thick without line broadening and loss ofresolution.

The MOD-metal flake mixture maintains its configuration during heating,and will decompose to form a well-bonded, well-resolved conductor at atemperature of approximately 200° C. Heating the deposit to atemperature just above about 300° C. surprisingly results in aconsolidation or sintering of the deposit to near theoretical densitywith an electrical resistivity of less than twice that of the bulk metaland excellent mechanical properties. Preferably, a metal in thecolloidal state is added to the mixture to provide a still lowertemperature for consolidation. The metal in the colloidal state ispreferably colloidal silver with a nominal particle size of 20nanometers in a α-terpineol solvent. The addition of a metal in thecolloidal state allows the mixture to be consolidate at a temperature ofabout 270° C.

The invention may be more readily understood by reference to thefollowing drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Scanning Electron Micrograph (SEM) of Aldrich Chemical Co.#32,707-7 silver flake showing its plate-like structure.

FIG. 2A is the test pattern used to measure the adhesion of conductorsprepared by the method of the present invention.

FIG. 2B is the test pattern used to measure the resistivity ofconductors prepared by the method of the present invention.

FIG. 2C is the test pattern used to confirm that the printed conductorscan be deposited without short circuits.

FIG. 3 is an SEM of a silver trace heat treated at 200° C. on anepoxy-glass substrate showing the unconsolidated nature of the deposit.Electrical conductivity was less than 1/4 that of bulk silver andadhesion and cohesion were inadequate.

FIG. 4 is an SEM cross section of a well-consolidated deposit showing anangled cross section with the Kapton substrate below and the surface ofthe deposit above.

FIG. 5 is a plot of resistivity in microhm cm vs. heat treatmenttemperature in degrees C. for a fifteen minute heating time, an inkcontaining silver neodecanoate, silver flake and α-terpineol and aKapton FN929 substrate. The critical temperature for consolidation isjust over 300° C.

FIG. 6 is a plot of resistivity in microhm cm vs. heat treatmenttemperature in degrees C. for a six minute heating time, an inkcontaining silver neodecanoate, silver flake and colloidal silversuspension and a Kapton FN929 substrate. The critical temperature forconsolidation is reduced to 270° C.

FIG. 7 is a plot of resistivity in microhm cm vs. heat treatmenttemperature in degrees C. for a five minute heating time, an inkcontaining silver flake and colloidal silver suspension, (no MODcompound) and a Kapton FN 929 substrate, showing low criticalconsolidation temperature but high resistivity.

FIG. 8 is a plot of resistivity in microhm cm vs. heat treatmenttemperature in degrees C. for a five minute heating time, an inkcontaining silver flake, silver neodecanoate and α-terpineol (nocolloidal silver) and a Kapton FN 929 substrate, showing low resistivitybut high critical consolidation temperature.

FIG. 9 is an SEM of a 0.010 inch (0.25 mm) diameter copper wire bondedto a silver interconnect with additional silver-MOD mixture showing anexcellent bond between the two metals and the well-consolidated natureof the silver deposit.

FIG. 10 is a replot of the data in FIG. 6 showing the relation betweenthe results obtained with the mixtures of this invention and those withthe mixture of the composition of U.S. Pat. Nos. 4,186,244 and4,463,030, with conventional conductive epoxies and with conventionalthick film inks. The resistivity is plotted on a logarithmic scale.

DETAILED DESCRIPTION OF THE INVENTION

During the course of this description, like numbers will be used toidentify like elements according to the different figures whichillustrate the invention.

The present invention relates to a composition formed of a mixture ofMetallo-Organic Decomposition (MOD) compound, and a metal powder, in theform of flakes and to processes to apply it to polymer-based substratesand convert it by heating to a temperature in the range of approximately200° C. to 350° C. to produce an electrical conductor on the substrateand to bond electrical components to the conductor.

The MOD constituent can be any compound in which a metal atom is linkedto an organic moiety through a heteroatom bond weaker than thecarbon-carbon bonds of the organic moiety. Examples of such MODcompounds are carboxylic acid metal soaps in which the metal atom isbonded via oxygen and which decompose readily with the evolution ofcarbon dioxide and hydrocarbon fragments, as well as the elementalmetal. Preferably, silver neodecanoate is used as the MOD compound inthe thick film composition.

Silver neodecanoate has the structure: ##STR1## Where R₁ +R₂ +R₃ =C₈ H₁₉

It will be appreciated that other metals and other organic constituentssuch as amines can also be used as the MOD composition. A typical aminecompound is gold amine 2-ethylhexanoate with the structure: ##STR2##

Other heteroatom linkages to the metals with sulfides and phosphides canbe used in the MOD component.

The composition of this invention decomposes and consolidates to awell-bonded metallic trace on a polymer-based substrate at a temperatureless than 350° C., preferably less than 300° C. and more preferably lessthan 275° C.

Preferably, the metal powder in the thick film composition is in theform of metal flakes with an aspect ratio, width divided by thickness,of between about 5 and about 50. Preferably, the aspect ratio is betweenabout 10 to about 20. An example, of a metal flake is a silver flakemanufactured by Aldrich shown at a magnification of 2000 in the scanningelectron micrograph in FIG. 1. Another metal flake useful for practiceof the present invention is a 15% silver-coated--400 mesh nickel flakeas supplied by Novamet, Inc.

The amount of metal flake used in the composition can be at least equalto the amount of the MOD compound by weight and less than about tentimes the amount of the MOD compound, preferably about four times theweight of the MOD compound. It has been found that the performance ofthe mixture is not sensitive to the amount of metal within a range oftwo to five times the MOD content.

The metal powder can be stabilized with a coating of surfactants, suchas stearic acid, to prevent premature agglomeration of the metalparticles. Such agglomeration can make the powder unfit for the purposeof this invention. Preferably, the amount of surfactant is minimized,and the use of strongly bonded surfactants which can interfere with theconsolidation of the metal powder is undesirable.

A colloidal metal suspension is preferably added to the mixture of thepresent invention to lower the critical consolidation temperature of thethick film. The colloidal material most preferably is substantiallysmaller in particle size than conventional metal powders with a meanparticle diameter in the range about 10 to about 40 nanometers.Preferably, the mean particle diameter is in the range of about 15 toabout 30 nanometers.

The metal content of the colloidal metal suspension can be at leastabout 10% by weight up to about 50% by weight. Typically, this materialcannot be handled as a dry powder and must be supplied as a stablesuspension in a liquid compatible with the final composition. Asuspension from Nanophase Technologies, Inc., 8205 S. Cass Ave. Darien,Ill. 60559 is an example of a colloidal metal suspension useful forpracticing the present invention. This suspension comprises 15% byweight of 20 nm diameter silver particles in an α-terpineol vehicle.α-terpineol is also used as the preferred vehicle for the presentinvention. It will be appreciated that other colloidal metal suspensionsin other vehicles which are known in the art can be used with theteachings of the present invention.

The vehicle used in the composition of this invention dissolves the MODcompound and suspends the metallic constituents of the mixture toprovide inks and pastes that can be applied by screen printing, stencilprinting, gravure printing or other direct contact printing processes.The vehicle has a low enough vapor pressure to maintain its consistencyon the screen for a period of more than an hours time. Preferably, thevapor pressure is less than about 0.001 atmosphere at ambienttemperature. The composition has a low enough viscosity to permit screenprinting. Preferably, the viscosity is about 10 Pa seconds (100 poise)at a shear rate of about 1000 per second. Preferably, the solvent isα-terpineol. Other vehicles such as butyl carbitol can also be used. Theamount of vehicle is dictated by the viscosity requirements of theprinting process. Preferably, the amount of the vehicle containing thecolloidal suspension is in the range of about 0.4 to about 1.5 times theMOD content of the composition by weight.

The composition of the present invention can be applied to a substrateby silk screening. Silk screening has the advantage of being convenientand widely used in the thick film industry. The mixtures can also beapplied by stenciling. Stencilling has the advantage of providing finerresolution than can be provided by silk screening. Intaglio or gravureprinting can be used to apply the composition of the present inventionin which an etched plate is filled with ink and pressed onto thesubstrate to transfer the desired pattern. The thick film of the presentinvention can also be applied by offset lithography in which the imageis made oil wetting to attract the oil-soluble ink and the background ofthe plate is made water wetting to repel the ink.

The mixtures of the present invention can be formed by weighing out theconstituents under dry nitrogen to avoid altering the surface state ofthe finely divided metal powders. The mixtures are blended to a smooth,homogeneous ink on a roll mill and are then ready for use.

Following application to the substrate, the composition of thisinvention is converted to a well-bonded, well-consolidated metalconductor by heating in an oven. For the noble metals silver, gold andplatinum, the MOD decomposition can be accomplished in air.Alternatively, the decomposition can be performed by heating in nitrogenwhich can improve conductivity of the silver traces.

For less noble metals such as copper and nickel, a special atmosphere isnecessary with an oxygen partial pressure below the equilibriumdecomposition pressure of the metal oxide to prevent oxide formation,but above the oxygen partial pressure in equilibrium with carbon and itsoxides to prevent carbon formation. Mixtures of hydrogen and water vaporor CO and CO₂ may be used, for example.

The decomposition of the composition of the present invention is rapidand complete. A fully sintered metal trace can be produced in a heatingtime of less than ten minutes, preferably less than six minutes, morepreferably less than five minutes.

The composition of the present invention can also be used to bondelectrical components to conductors. After a conductor has been appliedto a substrate as described above, the composition of the presentinvention can be applied to the electrical component and to theconductor and reheated for bonding the electrical component to theconductor. Alternatively, an electrical component can be adhered to anunheated circuit trace of the mixture of the present invention and thecircuit trace can be converted to metal and bonded to the substrate andthe component in a single oven treatment.

Unexpectedly, it has been found possible to sinter the MOD-metal flakemixture to a densified solid conductor at a temperature far lower thanthe melting point of the metal. Conventional sintering of metal requirestemperatures near the melting point. Copper powders under pressures ashigh as 100,000 psi (6700 atmospheres) do not begin to compact until thetemperature reaches 650° C. or 60% of the melting point. Conventionalsilver-based thick film pastes (melting point 962° C.) are sintered attemperatures above 850° C. at one atmosphere. The activation energy forsilver sintering is 46 kcal per mole. Sintering at 325° C. should beslower than at 850° C. by a factor of seventy million, but in factwell-consolidated, well-bonded conductors can be produced, presumablybecause the finely divided silver flake has a higher surface energy thanbulk silver, and in the MOD environment in which it is processed, it isfree of coatings or surface layers that would inhibit metal-to-metalcontact and consolidation. No preliminary drying step is needed in theformation of thick films. Also, the circuits do not need to be processedimmediately after screening. The addition of silver flake has solved theproblem of poor definition and inadequate thickness of the MOD mixtureby itself, while maintaining the MOD advantages of depositing a wellbonded metal image at a low enough temperature to be compatible withconventional printed circuit substrates.

The examples described below indicate how the individual constituents ofthe preferred mixture and the conditions of the process for applying itfunction to provide the desired result. The examples demonstrate that byusing the composition and processes of the present invention, solidmetal electrical conductors can be applied to conventional printedcircuit substrates without waste and with electrical performanceequivalent to thick film technology but at much lower temperatures. Theexamples will serve to further typify the nature of this invention butshould not be construed as a limitation in the scope thereof, whichscope is defined solely in the appended claims.

In the examples, the ink is screened onto the selected polymer-basedsubstrate using the test pattern shown in FIG. 2. The resulting screenedimages were heat treated in duplicate in a stationary infrared oven.They were introduced into a cold oven, and the radiant heater wasoperated at a power level sufficient to bring them to the desiredtemperature in 5-6 minutes. This simulates heating the circuits in aninfrared belt furnace. The solvent is driven off at a temperature belowabout 200° C. at which the MOD component begins to decompose. Continuedheating to a higher temperature mentioned in each example consolidatesthe metal conductor and bonds it to the substrate. It has been foundthat maintaining the peak temperatures for about one minute issufficient for consolidation and bonding the thick film of the presentinvention.

EXAMPLE I

A composition, labeled Ink B-2, was formed from the following mixture:

    ______________________________________    Silver neodecanoate 1.10 g    Silver flake, (Aldrich #32,707-7)                        0.88 g    Kerosene            0.48 g    ______________________________________

The constituents were blended by hand and screened onto a conventional0.062 inch thick FR-4 epoxy-glass circuit board substrates. The sampleswere heated in a stationary oven for twenty minutes at a temperaturemeasured as 185° C. This temperature could have been up to fifty degreeshigher based on subsequent measurements.

After heating, the MOD compound had completely decomposed leaving awell-defined and well-bonded image with a measured thickness of 0.0006inches (15 microns). Scotch Tape, Commercial Office Supply Division/3M.St. Paul, Minn., was applied to the image for removal of silver flake.The measured thickness of the bonded image was reduced to 0.0003 inches.The electrical resistance of the trace measured on a pattern with awidth of 0.4 mm and a length of 1.25 meters (3200 squares) was 115 ohms,corresponding to a resistivity of 54 microhm cm. A SEM photograph ofcomposition 1 is shown in FIG. 3. While the silver image 10 is firmlyadherent to the polymer substrate 12, the silver flake is poorlyconsolidated and full of holes and inclusions.

EXAMPLE II

A composition labeled Ink B-33 was formed from the following mixture:

    ______________________________________    Silver neodecanoate     3 g    Aldrich silver flake    12 g    (#32,707-77)    α-terpineol       1.8 g    ______________________________________

The constituents were weighed out under dry nitrogen and blended in theroll mill. Images were screened onto a DuPont Kapton FN 929 substratewith a semiautomatic screen printer and heated in a stationary oven asbefore, but with better temperature measurement and at successivelyhigher temperatures. By this technique it was found that there is acritical temperature above which the metal consolidates to a nearlysolid form, shown in the SEM cross section of FIG. 4. The sample isinclined with the Kapton substrate at 14, the surface of the silverdeposit at 16, the edge of the silver at 18, and a cross-section of thedeposit showing the degree of consolidation at 20. The measuredelectrical resistivities of the test conductors are shown as a functionof maximum oven temperature in FIG. 5. Duplicate samples were heated ina stationary oven in air, and the two resistivity values are plotted assolid and hollow squares. There is a sharp break downward in resistivityat an oven temperature just above 300° C. At higher temperatures, theelectrical resistivity of the printed and heat treated test conductorwas 3 microohm cm, compared to bulk silver at 1.59 microhm cm.

EXAMPLE III

By adding an even finer colloidal silver with a mean particle diameterof approximately 20 nanometers it has been found possible to lower thetemperature for consolidating the metal still further to 270° C., asshown in the following example.

A composition labelled Ink B-45 was formed from the following mixture:

    ______________________________________    silver neodecanoate     3 grams    silver flake, Aldrich Chemical Co.                            12 grams    Catalog No. 32,707    colloidal silver suspension*                            1.8 grams    ______________________________________     *Nanophase Technologies 15% by weight of 20 nanometer diameter colloidal     silver suspended in terpineol.

The ingredients were weighed out and mixed under dry, high puritynitrogen. The mixture was blended for fifteen minutes in a two roll millto prepare a smooth, uniform ink. The ink was screen printed to producea final thickness of silver of 10-15 microns on samples of DuPont Kapton300 FN 929. The printed pattern was a conductivity test pattern with aline width of 0.4 mm and a length of 1.25 m (3200 squares).

Three tests for mechanical properties were also performed on thesamples.

1) A mandrel test in which the sample was bent around a 1/8 inchmandrel.

2) A tape test in which tape was pressed down on the sample.

3) A crease test in which a 180° crease was formed in the sample.

The results of resistivity and mechanical properties vs. oventemperature were as follows:

    ______________________________________    Resistivity             Silver mass                       Temperature    (Microhm cm)             (Milligrams)                       (°C.)                                 Conditions    ______________________________________    5.26, 4.85             77, 75    228       Fails 1/8" mandrel test    5.21, 6.35             77, 84    256       Pass mandrel, fail tape    3.15, 3.24             75, 83    275       Pass tape, pass 180° crease    3.33, 3.39             84, 89    300       Pass tape, pass crease    2.11, 2.94             55, 73    327       Pass tape, 3/4 pass crease    ______________________________________

The resistivity results are plotted in FIG. 6. The resistivity droppedsharply at a temperature between 256° and 275° C., as was earlierobserved at a higher temperature. The lower temperature is attributed tothe presence of the colloidal silver, since the silver flake was fromthe same batch as used in the previous examples. The mechanicalproperties of the silver deposit improved at the same criticaltemperature as the electrical properties. At the lowest temperature, thedeposit flaked off the substrate when bent around a 1/8" diametermandrel. At 256° C. the deposit passed the mandrel test but failed the"Tape" test in which "Scotch" Tape is pressed down on the deposit andpeeled off. The deposit fails if it peels off the substrate. At 275° C.and above where the resistivity had decreased, the deposit passed thetape test, and withstood a 180 degree crease in the substrate, the 180°crease test is a severe test requiring adhesion and ductility in thedeposit.

The effect of the individual constituents of the mixture labeled inkB-45 are shown independently in the following two examples.

EXAMPLE IV

A composition labeled Ink B-50 was formed from the following mixture:

    ______________________________________    Silver Flake           12 grams    Colloidal silver suspension                           1.8 grams    (Nanophase)    α-terpineol      0.466 grams    ______________________________________

The constituents were weighed, blended and screened as above. Themixture without the MOD component is very different in consistency,being a stiff, noncohesive paste. Additional α-terpineol was addedduring roll milling to make the material more screenable. The extraliquid vehicle provided better images than previous mixtures withoutMOD, but the fired images were rougher and thicker than the imagesformed in Example III. Approximately twice as much silver was depositedper unit length of the conductor. The electrical resistivity is shown inFIG. 7.

The critical temperature for consolidation of the metal was between 280°C. and 290° C. and the physical properties of the conductors were goodat higher temperatures. The resistivity was significantly higher thanwhen the MOD compound was present. The results indicate that the MODcompound assists in producing a well-consolidated conductor with a lowresistivity, as well as acting as a surfactant and viscosity modifier toprovide high quality printed images.

EXAMPLE V

A composition labeled Ink B-49 was formed from the followingconstituents:

    ______________________________________    Silver neodecanoate   3 grams    Silver flake          12 grams    α-terpineol     1.8 grams    ______________________________________

The mixture was weighed, blended and screened as before. The electricalresistivity as a function of heating temperature is shown in FIG. 8. Itcan be seen that the critical temperature for consolidation is over 300°C., but that the absolute value of the resistivity is as low as with themixture formed in Example III. From this example, it is found that therole of the colloidal silver is to lower the critical temperature.

The conductors produced by the compositions and processes of the presentinvention are well-consolidated and well-bonded to epoxy-glass circuitboards as well as to Teflon-coated Kapton polyamide films as shown inthe following examples.

EXAMPLE VI

A screen ink with the same constituents as Ink B-45 described in ExampleV was prepared in the same way and screened onto substrate formed of anAllied Signal Advanced Materials FR 406 epoxy-glass rigid laminate. Thesame pattern was used but with a screen giving a nominal 17 micron thicktrace. The samples were heated in a nitrogen atmosphere in a beltfurnace with a six minute heating cycle, and with approximately a minuteduration at maximum temperature. The resistivity is shown below.

    ______________________________________    Resistivity (microhm cm)                      Maximum temperature (°C.)    ______________________________________    7.17, 7.25        260    5.69, 5.61        265    3.04, 3.18, 3.22 slight darkening                      270    ______________________________________

The same sharp reduction in resistivity as with Kapton substrates, shownin Example V is seen at 270° C. at which temperature the laminate stillretained its integrity. It was found that at 290° C. the substratedelaminated and discolored.

EXAMPLE VII

An adhesion pattern from the Military Specification MILS-13949H wasscreen printed on a Kapton FN substrate using the same mixture asExample III.

The conductors could not be peeled off the substrate without breakingthem, since, unlike conventional copper foil conductors, the cohesion ofthe deposited silver conductor is the same or less than its adhesion.The peel strength of the conductors was therefore measured by peelingthe substrate off the conductor after cementing the conductors to analuminum backing and scoring along the sides of the conductor strips.The 1/8 inch wide strips in the pattern were used. The range of resultsis presented below as Newtons force per meter of conductor width.

Peel Strength Tests--Screen Printed Kapton Samples

    ______________________________________    Sample no.    Newtons/meter    ______________________________________    B-40-1        250-400                  250-310                  250-525    B-40-2        310-525                  400-620                  250-620    B-40-3        220-460                  400-620                  400-620    B-40-4        460-560                  460-680                  250-525    ______________________________________

The results indicate significant adhesion of the deposited traces.Surface analysis of the separated traces and substrates shows fluorinein approximately equal amounts on each side of the separation,suggesting that the failure occurred at least partly in the FluorinatedEthylene Propylene (FEP) Teflon coating on the substrate.

EXAMPLE VIII

The ink described in Example I was screened onto FR-4 epoxy-glasslaminate for peel strength testing. As with Kapton, it proved impossibleto peel the conductors from the substrate without breaking them. Tomeasure the strength of the metal-substrate bond glass cloth wascemented to the conductors with high strength epoxy and scored along theconductor trace. When the glass cloth was pulled form the surface thefollowing peel strengths were recorded:

Peel Strength Tests--Epoxy-Glass Samples

    ______________________________________                              Majority of    Sample No.              Newtons/Meter, Range                              Data and Remarks    ______________________________________    B-9-1     1390-2320              775-1700        Pulled the board apart              1080-2630       1500-1600,                              removed metal              620-2630        1500-1600,                              pulled board apart    B-9-2     620-2160        1550-1850              930-2160        1235-1550              930-2475        1235-1550              930-1850        930-1235    ______________________________________

In all cases the metal-epoxy bond was far stronger than the metal-Teflonbond of Example III. The metal was so firmly anchored that in some teststhe substrate actually delaminated before the bond broke.

In addition to providing conductors bonded to polymer substrates, thethick film mixtures of the present invention can also be used to bondelectrical leads from components to the conductors, thereby facilitatingassembly of the circuit as well as making the circuit board. Thefollowing example illustrates this application.

EXAMPLE IX

An ink formed as in Example III was prepared screened and heated asabove. Additional ink was used to secure a copper wire 22 with adiameter of 0.010 inch (0.25 mm) to one of the circuit traces. Thecircuit was reheated with the same time-temperature profile and thecopper wire was sectioned and photographed with the SEM. The result isshown in FIG. 9. The silver deposit was consolidated with no evidence ofthe interface between the first layer of silver 24 and the second layer26. The bond to the copper wire 28 was intimate and without voids orinclusions, despite the absence of precautions such as fluxes orprotective atmospheres. The wire was solidly bonded, and when pulled offthe circuit, it removed some of the silver with it.

The relationship of conductors produced from the mixtures of the presentinvention to those available in the prior art, and to U.S. Pat. No.4,463,030, is shown in FIG. 10. Conductors 30 of the present inventionat low temperature have less resistivity than conductors 32corresponding to the '030 patent and conventional conductive epoxies 34.They can achieve equally low resistivities at dramatically lowertemperatures than conventional thick film pastes 36. The mixtures of thepresent invention make possible the creation of practical printedcircuits in a region of high electrical conductivity and low processingtemperature hitherto unattainable.

The compositions and the processes for applying them to polymersubstrates of the present invention have the advantage of providingnovel capabilities for creating electrically conductive patterns andcoatings on various plastic substrates. The conductors of the presentinvention are pure metals or alloys with no admixtures of polymer orglass binding agents, and therefor provide an unexpected level ofelectrical conductivity. The compositions can be applied in layers of anarbitrary thickness offering significant current carrying capacity. Thecompositions can be applied only where needed and only in the amountdesired in the substrate by various printing technologies with nohazardous waste generation. The present invention is applicable to anumber of metals and all the conventional printed circuit substratematerials. The compositions of the present invention can also be used tocreate electrical resistors, dielectrics, decorative coatings andoptical coatings.

While the invention has been described with reference to the preferredembodiment thereof, it will be appreciated by those of ordinary skill inthe art that modifications can be made to the structure and elements ofthe invention without departing from the spirit and scope of theinvention as a whole.

I claim:
 1. A method for depositing an electrical conductor on apolymer-based substrate comprising the steps of:a. mixing ametallo-organic decomposition (MOD) compound with a first metal powderin an amount of about 1 to about 10 times the amount of the MOD compoundby weight to form a mixture, and then mixing said mixture with either(i) an organic liquid vehicle or (ii) with a second metal in a colloidalsuspension in said organic liquid vehicle, said second metal having amean particle diameter of about 10 to about 40 nanometers; b. printingsaid mixture on said polymer-based substrate; and, c. heating saidmixture to a temperature below 450° C. for a time period of less than 5minutes,wherein said printed mixture is converted into a solid metalconductor on said polymer-based substrate, said solid metal conductorhaving a thickness greater than 1 micron and a resistivity of less than5 times the bulk metal resistivity of said first metal.
 2. The method ofclaim 1 wherein said first metal is selected from the group consistingof: copper, silver, gold, zinc, cadmium, palladium, iridium, ruthenium,osmium, rhodium, platinum, iron, cobalt, nickel, indium, tin, antimony,lead, and bismuth.
 3. The method of claim 1 wherein said second metal isselected from the group consisting of: copper, silver, gold, zinc,cadmium, palladium, iridium, ruthenium, osmium, rhodium, platinum, iron,cobalt, nickel, indium, tin, antimony, lead, and bismuth.
 4. The methodof claim 1 wherein said MOD compound is selected from the groupconsisting of: silver neodecanoate, gold amine 2-ethylhexanoate,platinum amine 2-ethylhexanoate, carboxylic acid soaps, amines, andcompounds with hetero atom linkages of sulfides and phosphides.
 5. Themethod of claim 1 wherein said printing is performed by a methodselected from the group comprising: silk screening, stencil printing,gravure printing and direct contact printing.
 6. The method of claim 1wherein, said mixture is heated to a temperature in the range of about200° C. to about 350° C.
 7. The method of claim 1 wherein, said mixtureis heated for a time period of less than 2 minutes.
 8. The method ofclaim 1 wherein, said mixture is heated for a time period of less than 1minute.